IDYLLWILD, Calif. — Residents of another 700 homes were advised to retreat to safety on Friday as crews fighting a wildfire in the mountains above Palm Springs grew increasingly concerned about the possibility of unstable weather and erratic winds.

The voluntary departures by people in Pine Cove, on the fire’s western flank, came in addition to mandatory evacuations involving 6,000 others who spent a third day away from home as the fire spread in three directions.

The blaze in the San Jacinto Mountains has expanded to roughly 39 square miles and was 15 percent contained, U.S. Forest Service spokeswoman Kate Kramer said.

Some communities on the eastern edge of the fire were reopened to residents, but about 4,100 homes remained under potential threat.

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The fire was only about two miles from Idyllwild on its western flank and the same from Palm Springs, below on the desert floor. However, it was burning relatively slowly with the most active area south of town.

An enormous plume of smoke could be seen from Palm Springs.

A storm front headed toward the region could provide some relief with cooler weather and a chance of rain — but it might make the situation much more volatile, fire spokesman Capt. Mike Lindbery said.

Combined with hot air on the ground, the unstable air could create a strong updraft that draws smoke high into the atmosphere.

If the smoke column rises too high, moisture at the top could freeze and the weight of the ice could cause the column to collapse, creating a powerful downdraft in all directions, Lindbery said.

“We’re very concerned because this is the condition in the past that has definitely caused big firestorms and the death of citizens and firefighters,” he said.

“It makes it so there are not a lot of places that are safe.”

Popular campgrounds, hiking trails and a 30-mile section of the Pacific Crest Trail that runs from the Mexican border to Canada, remained closed. A small stretch of a local highway was also closed.

Some 3,300 firefighters, aided by nearly 30 aircraft, battled the fire, which stretched from 4,000 feet to 9,000 feet along the mountains. As a result, crews could be working in temperatures ranging from a comfortable 75 to a scorching 110 degrees.

The arriving storm front could bring a 20 percent chance of rain and 15-24 mph winds with gusts to 40 mph that could push the flames in erratic ways.

The fire, which began Monday, has burned six homes and mobile homes, one cabin, and more than a dozen other buildings. One home also was damaged.

Idyllwild resident Steve Hamlet had been watching a growing smoke cloud since Monday and evacuated his home with his 12-year-old grandson as flames crept closer.

On Friday, he worried whether he had packed the right things — and worried about his wife, who had stayed behind.

“Do I take pictures? Do I take bills? Do I take legal documents? Or do I just take my grandson and I?” Hamlet said at a local evacuation shelter.

He and his grandson tried to check on his wife Thursday but were turned away at a road block after driving 25 miles up the mountain, he said.

“She knows enough when it’s time to leave,” he said.

Like Hamlet, more than 100 evacuees, including campers rousted from spots in the danger zone, spent the night at shelters located at area schools.